1. Field of the Invention
The present invention relates to a dot line printer having a reciprocally movable hammer bank wherein a predetermined number of dot lines are printed with each of forward and backward movements of the hammer bank and printing of one character line is completed by reciprocating the hammer bank a predetermined number of times.
2. Description of the Related Art
FIG. 1 shows a conventional dot line printer. In this printer, continuous print paper 4 is used which is provided with uniformly-spaced perforations which are drivingly engaged by a sheet feed mechanism such as a pin feed tractor 5 to intermittently move the paper 4 in a sheet feed direction F. A platen 7 is rotatably supported on a printer frame (not shown) for supporting the print paper 4.
The hammer bank 1 is reciprocally movably supported in a printer housing (not shown) and is in confrontation with the paper 4. The hammer bank 1 accommodates therein a plurality of dot print hammers juxtaposed at an equi-distant pitch along a line extending in a columnar direction C perpendicular to the sheet feed direction F. The print hammer is in the form of an elongated leaf spring having an upper end to which a pin is attached for making an impression of a dot on the print paper 4 through an inking ribbon. The pins of the print hammers are displaced at an equi-distant pitch in the sheet feed direction F so that an N-number (N being an integer) of dot lines are printed with each of forward and backward movements of the hammer bank 1. The print paper 4 is fed intermittently by an amount corresponding to the N-dot lines in the paper feed direction F upon completion of printing of N-dot lines.
The inking ribbon 3 is transported by a ribbon transport mechanism 6 and passes between the print paper 4 and the hammer bank 1. The inking ribbon 3 is moved in the columnar direction C in the portion between the print paper 4 and the hammer bank 1. A drive mechanism (not shown) for driving the print hammers is housed in the hammer bank 1.
A shuttle mechanism 2 is provided for reciprocally moving the hammer bank 1 in the columnar direction C. The shuttle mechanism 2 includes a shuttle motor and a cam 2. To print dot matrix characters or graphics on the print paper 4, the print hammers are selectively driven by the drive mechanism while the hammer bank 1 is reciprocated with forward and backward movements by the shuttle mechanism 2 across the print paper 4.
When the hammer bank 1 makes one way movement, e.g. from the leftmost and to the rightmost end of the hammer bank reciprocating path, N-number of dot lines are printed simultaneously. Upon printing the first N-number of dot lines, the print paper 4 is fed N-number of dot lines in the sheet feed direction F. Then the hammer bank 1 moves backwardly from the rightmost and to the leftmost end. During the backward movement of the hammer bank 1, another N-number of dot lines are printed. This procedure is repeated until desired characters or graphics are printed. The movement of the hammer bank 1 from one end (for example, the rightmost end) to the opposite end (for example, the leftmost end) or vice versa, will hereinafter be referred to as "one scan".
Next, a method of printing a character with the line printer shown in FIG. 1 will be described while referring to FIG. 2. In this example, 8 dot lines can be printed simultaneously with each scan of the hammer bank 1 and the character is expressed by a 24-by-24 dot matrix having 24 dot printable positions in row and column. Therefore, three scans are required to complete printing of one character line. More specifically, during the first scan of the hammer bank 1 moving, for example, rightward, let through 8th dot lines of a first character line are printed. While the hammer bank 1 is reversing at the rightmost end of the hammer bank moving path, the print paper 4 is fed an amount corresponding to eight dot lines. During the second scan of the hammer bank 1 moving leftward, 9th through 16th dot lines are printed. During the hammer bank reversing time at the leftmost end, the print paper 4 is again fed an amount corresponding to eight dot lines. Thereafter, 17th through 24th dot lines are printed during the third scan. This finishes printing of the first character line. The direction of the hammer bank 1 is then reversed and the hammer bank 1 returns to the leftmost end from which printing of the first scan of the first character line starts. During the interval the hammer bank 1 returns to the initial position, the print paper 4 is fed to reserve an interline space between two successive character lines. Printing is performed by repeating the above-described sequence of processes.
In the dot line printer of the type as described above wherein a predetermined number of dot lines are printed simultaneously with each scan of the hammer bank 1, the pins of the print hammers are arranged at a predetermined equi-distant pitch in the sheet feed direction F. To make an impression of two continuous dots of, for example, 1/168 inch in the sheet feed direction F, the pins of the print hammers must be arranged at an equi-distant pitch of 1/168 inches in the sheet feed direction F. Therefore, the precision of dot positions in the sheet feed direction F printed during the same scan of the hammer bank, for example, the precision of the dot positions printed on the 1st through 8th dot lines, is determined solely by the precision in the arrangement of the print hammer pins in the sheet feed direction F.
However, in the 8-dot-line printable hammer arrangement, the space between the dots in the eighth dot line of the first scan and the dots in the first dot line of the second scan is determined not only by the positions of the print hammer pins arranged in the sheet feed direction F but also an accumulative error of the sheet feed deviation and an operational difference between forward and backward movements of the hammer bank 1 by the shuttle mechanism 2. Accumulative error can produce greater variation than the precision at which dot print hammers are positioned, but is mechanically unavoidable. Therefore, ensuring accurate positioning between printed dots of different scans is difficult using conventional technology.
FIG. 3 shows a dot pattern produced by two perfectly executed scans (i.e., with zero accumulative error) wherein N-number of dot lines are printed in each scan. As can be seen, all adjacent dot lines are separated by the same distance because a predetermined pitch is maintained between dot lines during both the sheet feed operation in the sheet feed direction F and the shuttle operation in the columnar direction.
FIG. 4 shows a dot pattern produced when the interval between the first scan and the second scan broadens (hereinafter referred to as "separation in the positive direction"). When this type of accumulative error occurs, the interval between the n-th dot line (n(dl)) of the first scan and the first dot line (1(d1)) of the second scan is greater than the predetermined pitch. This can cause a gap or white line to appear between printed dot lines.
FIG. 5 shows a dot pattern produced when the interval between the first scan and the second scan narrows (hereinafter referred to as "separation in the negative direction"). When this type of accumulative error occurs, the interval between the Nth dot line (N(d1)) of the first scan and the first dot line (1(d1)) of the second scan is less than the predetermined pitch. This can cause adjacent dot lines that should be separated to come in contact or overlap. In the situation shown in FIG. 5, even if the accumulative error is small, printed characters can appear squashed or the width of lines extending in the feed direction F can appear nonuniform.